A Quad-Fast-MAS probe for Dramatically Improved Biomolecular Structure Determinations Abstract The last 15 years have seen steady progress in applying magic angle spinning (MAS) NMR to an increasingly wide range of applications in structural biology. However, the methods are all far from routine and often require 10-80 mg of a concentrated sample that is extremely difficult to isolate and prepare suitably. The Holy Grail i solids NMR would be the ability to successfully utilize the powerful suite of NMR acquisition and automated structure determination protocols developed for solution NMR, which rely mostly on 1H-detected triple- and quad-resonance schemes (as such generally permit 8 or 30 times higher S/N than direct detection, for 13C and 15N respectively) with solid samples of 1-10 mg. Four-channel multinuclear probes with gradients have been the workhorse in solution NMR for decades, but quad-resonance solids probes have not been available - they have been perceived to be impractically difficult to design and build. This proposal seeks funding to develop, build, and test a prototype H/X/Y/Z HR-fast-MAS probe based on a novel single-coil rf circuit optimized for 1H detection and suitable for use at fields from 7-30 T and rotor diameters from 0.5-5 mm. The Phase-II probe will be compatible with automated sample exchange, pulsed-field gradients (PFG), and sample temperatures from 90 K to 400 K. Moreover, it will be essentially devoid of background signals for all the primary nuclides (1H, 31P, 13C, 2H, 15N, and 17O). Order-of-magnitude improvements in spectral resolution have been demonstrated for 1H-detected methods in solids from the combination of improved sample perdeuteration, optimal protonation of exchangeable sites, faster spinning, and higher polarizing fields, seeing typical 1H linewidths in rigid proteins decrease from ~0.4 ppm to ~0.04 ppm. Analysis suggests that a substantial portion of the remaining broadening is from J-couplings to the deuterons (which is not averaged by MAS) and spinner-dependent effects - thermal gradients, axial vibration, and magnetism. Calculations suggest 2H J-couplings contribute 5- 10 Hz to 1H line broadening, and available data suggest the probe-limited resolution in commercially available fast-MAS probes has contributed another 6-25 Hz. A 4-channel HR-MAS probe that permits simultaneous decoupling of 13C, 2H, and 15N, achieves >40 kHz MAS rotation with order-of-magnitude lower thermal gradients, and is capable of 2 Hz resolution on liquids 1H resolution, is expected to enable 1H linewidths below 0.01 ppm on most of the residues in rigid proteins at 900 MHz and above. The novel circuit will also be tunable to virtually all combinations of interest, such as 1H/13C/2H/15N, 1H/31P/13C/2H, 1H/31P/7Li/13C, 1H/27Al/29Si/17O, and 1H/13C/29Si/103Rh, thereby making it also invaluable in such areas as metabolism, neurology, materials science, catalysis, and sustainable energy.